In an aircraft axial-flow gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is combusted, and the resulting hot combustion gases are passed through a turbine mounted on the same shaft. The flow of gas turns the turbine by contacting an airfoil portion of the turbine blade, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward. There may additionally be a turbofan that drives a bypass flow of air rearwardly to improve the thrust of the engine.
The compressor, the turbine, and the turbofan have a similar construction. They each have a rotor assembly including a rotor disk and a set of blades extending radially outwardly from the rotor disk. The compressor, the turbine, and the turbofan share this basic configuration. However, the materials of construction of the rotor disks and the blades, as well as the shapes and sizes of the rotor disks and the blades, vary in these different sections of the gas turbine engine. The blades may be integral with and metallurgically bonded to the disk, forming a BLISK (“bladed disk”), or they may be mechanically attached to the disk.
The turbine disks and blades are subjected to high loadings during service, and the nature of the performance-limiting consideration varies with radial position. The periphery of the disk is at a higher temperature than the hub of the disk. The performance of the periphery portions of the turbine disks and the turbine blades are limited by creep loading and defect tolerance. The performance of the hub portions of the turbine disk are limited by tensile and cyclic loading. Nickel-base superalloys are the best available material compositions for use in the turbine blades and disks.
The metallurgical grain sizes are also selected to meet the property requirements. Turbine airfoils are often cast using directional solidification to achieve either preferred grain boundary orientations or to eliminate the grain boundaries entirely. Airfoils may also be cast hollow or with integral cooling passages. The forged grains along the disk periphery are preferably relatively coarse to resist creep deformation. The forged grains of the central disk hub are preferably relatively fine for good tensile and fatigue strength. A number of different metallurgical processing techniques are used to produce the different types of microstructures required in the single BLISK or bladed-disk article. Different forging processes, heat treatments, and thermo-mechanical processing are used for the different parts of the disk.
These manufacturing techniques, while operable, are difficult to apply in production practice. The disks are relatively large in size, often several feet across, and it is difficult to achieve a highly controlled microstructure over this large area. The processing must allow the development of the desired precipitation-hardened microstructure, while also achieving the required grain size distribution. The problem is even more acute when the rotor assembly is a BLISK, where the heat treatment of the disk must be compatible with the bonding process of the blade to the disk. The airfoil bonding process is often performed using diffusion-dependent processes which benefit from high temperature exposure that are incompatible with critical metallurgical temperatures which cannot be exceeded if the fine-grain central disk hub is to be realized.
There is a need for an improved approach for preparing a rotor assembly for an axial-flow aircraft gas turbine. The approach must achieve the required microstructures in a production setting. The present invention fulfills this need, and further provides related advantages.